Chaff was one in the first forms of countermeasure employed against radar. It usually consists of a large number of electromagnetic dispersers and reflectors, normally arranged in form of strips of metal foil packed in a bundle. When they are released by an aircraft or distributed by rockets launched by a ship, most of the strips of foil which constitute the chaff bale are dispersed by the effect of the wind and become highly reflective clouds.
Chaff is a relatively slow target. Its vertical descent is determined by the force of gravity and for the properties to resist advance presented by the strips of individual leaves. Chaff was a very effective countermeasure when using slow bomber aircraft during the Second World War. Chaff is usually employed to foil or to confuse surveillance and tracking radar. Miscellaneous reference information on radar chaff can be found in M. I. Skolnik's “Introduction to Radar Systems”, McGraw-Hill, London, 1981.
To date various inventions related with chaff have already been presented. Many of them are related with the distribution and ejection of chaff (see for example the patents with publication numbers EP0246368, EP0036239, EP0036239, U.S. Pat. No. 4,597,332, U.S. Pat. No. 4,471,358, U.S. Pat. No. 5,835,682) or with the materials and composition used in the reflectors and dispersion particles (see for example the patents with publication number U.S. Pat. No. 5,087,515, U.S. Pat. No. 4,976,828, U.S. Pat. No. 4,763,127, U.S. Pat. No. 4,600,642, U.S. Pat. No. 3,952,307, U.S. Pat. No, 3,725,927). Nevertheless, little attention has been paid to the design of the shape of the dispersers which form the cloud. A design for a disperser in sword form is described in the Patent GB2215136, which provides a way whereby the dispersers descend rotating by the effect of gravity, facilitating a complex radar cross-section (RCS) which can confuse systems with Doppler radar.
The heart of the present invention lies in the geometry of the dispersers or reflectors which improve the properties of radar chaff.
Some of the geometries employed in the present invention are already related with some forms expounded for antennas. Multilevel and space-filling antennas are distinguished in being of reduced size and having a multiband behaviour, as has been expounded already in patent publications WO0154225 and WO0122528, respectively.
Nevertheless, it is to be stressed that the dispersers used in the present invention are not antennas, and that the features required of antennas are different with regard to those required by radar chaff. Antennas are used to transmit and receive associated signals to or from a transceiver by means of a transmission line or a radio frequency network.
Also, antennas are composed of several parts, like the radiating elements, the ground planes or ground references, as well as connectors for input and output terminals. The dispersers presented in the present invention are not used to receive or transmit signals and are not associated with any transceiver, nor do they comprise a assembly of complementary elements like ground planes, connectors, etc. The main technical characteristics sought in the design of an antenna are gain, radiation pattern and impedance. In radar chaff it makes no sense to design for gain or impedance, since dispersers have no terminal by which to define an impedance and, since they are not an instrument for receiving or transmitting, the gain parameter is of no sense. The main electrical characteristic of a radar chaff disperser is its radar cross-section (RCS) which is related with the reflective capability of the disperser, and which cannot be anticipated by the characteristic parameters of the antennas. The chaff dispersers expounded in the present invention are mainly electromagnetic reflectors constituted of a conducting, semiconducting or superconducting material with a new geometry which improves the properties of the chaff. The new geometry facilitates a large RCS compared with dispersers presented in previous inventions having the same size; surprisingly the RCS is equivalent to that of conventional dispersers of greater size.
A review of the state of the art in radar chaff reveals totally different geometries for chaff dispersers (mainly rectilinear strips and meshed fibres) which endeavour to resolve packaging density by means of the materials used in the chaff, mainly dielectric fibres with a fine metallic cladding. In the present invention, the distinctive sizes for the new geometry presented are combined with a type of surface which provides a better aerodynamic profile which permits an improvement in the suspension properties of the whole radar chaff cloud. Clearly, since the essence of the invention resides in the particular properties of reflection of the new geometries presented for the chaff dispersers, these new geometries are compatible and can be combined with any of the materials and manufacturing techniques described in the state of the art.